(a) Field of the Invention
The present invention relates to a pulse laser for regeneration of optical clock pulses that is suitable for 3R regeneration—retiming, reshaping, and reamplifying—to restore optical signals distorted while being transmitted via an optical cable. More specifically, the present invention relates to a high frequency optical pulse source capable of generating stable optical pulses using self mode locking in a multi-electrode distributed feedback laser diode.
(b) Description of the Related Art
The prior art of the pulse laser related to the present invention will now be described, as follows.
An element comprised of two active layers with a saturated absorption layer can generate optical pulses of approximately 5 GHz, but it is difficult to acquire faster optical pulses because the life time of carriers is limited to about 1 ns (Ref. 1: P. E. Barnsley, IEE Proc. J, vol. 140, 1993).
As a multi-electrode distributed feedback pulse laser that has recently been proposed in an attempt to overcome the problem, a two-electrode distributed feedback laser can generate GHz-level optical pulses by regulating input currents of electrodes (Ref. 2: M. Mohrle, IEEE, Phtonics Technol. Lett. Vol. 4, 1992). But this laser system generates optical pulses limited to 15 GHz, and it requires a phase control section integrated on the element for the purpose of guaranteeing high repeatability and controllability, because the phase of gratings varies at random from element to element in a two-electrode distributed feedback (DFB) laser.
In another example, optical pulse generation and injection locking in a section of greater than 40 GHz is carried out using beats in an element with a phase control section integrated therein (Ref. 5: M. Mohrle, IEEE J. Quantum. Electron. Lett. Vol. 7, 2001).
Contrarily, a laser diode with incident light output from an external resonator can generate two compound-cavity modes and achieve self mode locking by the contention of these modes (Ref. 6: A. A. Tager et al., IEEE J. Quantum. Electron. Lett. 30, 1994).
The latest technology relates to optical pulse generation from an optical element that includes, in one chip, a distributed feedback semiconductor laser diode, a phase control section, and an amplifier having a cleaved section without a non-reflecting coating (Ref. 7: S. Bauer et al., IEEE J. Quantum. Electron. Lett. 38, 2002).
However, the above-mentioned technologies tend to cause mode hopping or multiple modes in DFB laser section according to the phase change in actual applications, adversely affecting the generation of optical pulses and a stable frequency change.